The present invention relates to a dielectric line to be used for transmitting energy of electromagnetic waves such as millimetric or submillimetric waves and, more particularly, to a dielectric line equipped with means for emitting electromagnetic waves directly from one end portion thereof into space without the use of a metallic waveguide.
When a metallic waveguide is used as a guide for the passage of microwaves so that electromagnetic waves may be radiated from the end portion of a dielectric transmission line, it is current practice to attach a metallic antenna having a horned opening to the end portion. In order to radiate plane waves, systems such as a lens antenna system are used, in which the metallic antenna is used as a primary radiation antenna and a dielectric lens is used in the advancing direction of the electromagnetic waves, or a reflector antenna system in which a reflecting mirror of metal is used, or a Cassegrain antenna system, in which two reflecting mirrors are used, is employed.
In accordance with the development of a semiconductor for transmission of electromagnetic waves in the millimetric wavelength range, practical application of radio communications, radar systems and other applications have increased and a dielectric transmission line has been used as a waveguide in and between apparatus in such applications. The prior dielectric line, as indicted in its entirety at numeral 1' in FIG. 15 is constructed of a central core 2 made of a porous plastic material having a relatively high dielectric constant, and a cladding 3 coaxially enclosing the core 1' and made of a plastic material having a relatively low dielectric constant so that the electromagnetic wave energy may be confined in and propagated mainly through the core 2. Reference numeral 4 indicates an insulating protective layer covering the outer circumference of the cladding 3.
The dielectric line having the construction described above has a variety of advantages such as ease of working or connection with other parts, and ample flexibility because it has less insertion loss and a larger size for high-frequency waves than a metallic waveguide. As a result, such a dielectric line has been used more and more frequently.
When electromagnetic waves are to be radiated from the end portion of the aforementioned dielectric line, it is current practice to connect a metallic waveguide and a metal horn to the dielectric line through a connector called a "launcher". The reasons therefor are as follows:
(1) It is possible to use existing and completed electromagnetic wave radiation techniques which use a metallic waveguide and a metal horn;
(2) In the radiation or transmission of electromagnetic waves, the launcher preserves the phase center that is to be transmitted from dielectric line to a metallic waveguide or horn via the launcher;
(3) The end portion of the dielectric line can be firmly fixed because it is fixed at the position of the metallic guide tube. The electromagnetic waves in the metallic guide tube are not disturbed even if the guide tube has its outside fixed by means of a fixture, because there is no electromagnetic wave present outside of the guide tube.
If a metallic waveguide is connected to the dielectric line, however, registration of the transmission constant at the connecting portion is so difficult as to cause increase in insertion loss, the deterioration of attenuation due to reflection, the displacement of phase planes, and other problems.
If the dielectric line is twisted to change the plane of polarization, there arises another problem in that registration with the metallic waveguide deteriorates making the frequency band narrower.
Investigations of means for radiating electromagnetic waves from the end portion of a dielectric line have led to the present invention, which is characterized not by connecting a metallic waveguide as in the prior art, but by disposing, in association with one end portion of the dielectric line, means for extracting the electromagnetic waves radiated in the form of a plane front from said one end portion of the dielectric line.